Lens arrays allow the generation of a number of different types of optical effect. For example, an array of lenses focussed on underlying image elements at the focal plane of the array can generate integral images which appear to be three dimensional, which move, change magnification or morph as the viewing angle changes, or which have apparent depth outside the plane of the lens array. A further effect can be achieved by interleaving two or more images underneath the lenses, for example in strips underneath a plurality of cylindrical lenses, so that the viewer sees different images as the viewing angle changes. Such visual effects are useful in a number of applications including displays, promotional materials, collectable articles and as optically variable devices in security documents.
Lens arrays are generally manufactured from transparent polymeric materials to produce a sheet of material, referred to herein as a lenticular sheet. The pattern of the lenslets forming the array is embossed or otherwise formed on one side of the sheet, and the opposing side of the sheet is formed into a flat, generally gloss-type surface. The image elements are applied to or placed at the flat surface and may be formed, for example, by printing or by a laser marking process. The sheet material is commonly manufactured as a monolayer, but multilayer methods are also used.
The image elements may comprise printed dots. In one process, prior to printing, a continuous image representing the desired final print on the flat surface is converted to a halftone image. After printing, the halftone image will appear as a plurality of printed dots on the flat surface.
The thickness (usually termed the gauge thickness) of the lenticular sheet has traditionally been determined by the focal length of the lenslets, such that incoming light rays substantially focus at the flat surface of the sheet. This design is chosen to take advantage of the so-called sampling effect. The sampling effect ensures that a dot printed at the focal length of the lens will appear to an observer at a particular viewing angle as a line across a cylindrical lens, and will appear to fill the entire lens area for a non-cylindrical lens. Therefore, an observer cannot distinguish two adjacent dots inside a single lens at a particular viewing angle.
In some cases, the material thickness and lens frequency (or pitch) of the lenslets may be pre-selected according to the needs of the end product, as well as the gauge limitations of the sheet material manufacturing process. The lens radius of curvature is then determined based on additional parameters such as the refractive index and Abbe number of the polymeric material used, so as to focus incoming light rays substantially at the flat side of the sheet.
The recent trend in the art has been to produce thinner lenticular sheets so as to reduce manufacturing costs, while at the same time broadening the potential applications of optical effect articles. However, a thinner lenticular sheet generally requires a higher lens frequency in order to produce a focussed image. For example, a material produced with a gauge thickness of 85 microns in polyester would require a lens frequency of approximately 224 lenslets per centimeter. Printing optical effect imagery on these high frequency microlens arrays is particularly challenging, and severely limits the type of effect that can be achieved and the type of press and prepress methods than can be used. Furthermore, a high rate of waste material often results as very high line screens must be utilised and very accurate colour-to-colour registration becomes critical. These problems have meant that use of very high frequency lenticular sheet material has heretofore been limited.
One attempt to overcome the above problems is described in U.S. Pat. No. 6,833,960. Lenses are formed as hemispheres on a substrate using curable resins in a printing press. In this method, it is not possible to form the lenses on the substrate at their focal point. The lenses are thus substantially off-focus and this nullifies the sampling effect. The imagery produced by the method is thus substantially blurred.
Another method is described in U.S. Pat. No. 6,989,931 and includes a composite image comprised of printed stripes viewable through a lenticular screen from a first angle, with an object or image placed a distance behind the lenticular screen viewable through transparent stripes at a second angle. In one embodiment, a lenticular material thinner than its focal length can be used since the stripes printed on the flat side of the lenticular material do not contain multiplexed optical effect imagery. The lenslets need not possess the resolving power required for traditional optical effect imagery, but instead view only one half of the area behind each lenslet. However, use of this method for multiplexed lenticular imagery or intricate moiré effects can produce severe blurring.
Accordingly, there is a need for a method of reducing the gauge thickness of a lens array without introducing substantial blurring or other objectionable image artifacts.
In some circumstances it may be desirable to manufacture a lenticular sheet at a particular gauge thickness. If this is the case, it may then be desirable to reduce the lens frequency, i.e. to increase the width of each lenslet, in order to maintain image quality in view of the constraints of the printing process (or other process for forming the image elements) to be used. It is therefore desirable to provide a lens array and a method which allows a lower lens frequency to be used without introducing substantial blurring or other objectionable image artifacts.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is solely for providing a context for the disclosed embodiments. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the disclosed embodiments as they existed in Australia before the priority date of each claim in this application.